Syd Field's Screenplay: The Foundations of Screenwriting is a very good resource but you'll want to expand beyond it.

I've found one of the best ways to learn about screenwriting is to read a lot of scripts (just like one of the best ways to learn about writing for kids is to read a ton of kidlit). You can find a few websites that have scripts for popular (and not so popular) movies if you do a search.

I'll second Save the Cat, which is also a great resource for novel writing. I've heard that many film execs use that structure as a guideline for reviewing scripts. Bonus, it's easy to comprehend and apply. Good luck!

As a former TV scriptwriter and current screenwriting teacher, I highly recommend you begin your screenwriting education with two excellent books: 1) The Screenwriter?s Bible (5th edition) ? David Trottier; and 2) Writing Movies: The Practical Guide to Creating Stellar Screenplays - Gotham Writers' Workshop.Both of these books are written in easy-to-understand language and offer lots of great examples that make writing/storytelling/filmmaking concepts easy to comprehend. You'll never watch movies the same way again. Good luck! :